1. Field of the Invention
The present invention relates to an engine equipped with a variable valve timing mechanism, which may be called hereinafter “variable valve timing mechanism-equipped engine” particularly capable of improving machinability and mountability of the variable valve timing mechanism with enhanced freedom of arrangement of an oil passage and a hydraulic control valve of the variable valve timing mechanism of the engine to be mounted on a vehicle (vehicle body).
2. Description of the Related Art
Engines to be mounted on vehicles such as automobiles include an engine equipped with a variable timing mechanism (so-called variable valve timing mechanism-equipped engine), each in which a hydraulic actuator is attached to a camshaft, and the hydraulic actuator is operated by a working oil (i.e., hydraulic oil) supplied from an hydraulic control valve attached to a chain case to thereby change rotational phase of the camshaft with respect to the crankshaft.
One common mounting method for a hydraulic control valve of a variable valve timing mechanism is a method of directly inserting a hydraulic control valve into a hydraulic control valve-attaching portion provided in a chain case. In this method, a method of forming an oil passage is generally performed in a manner such that an oil passage on a cylinder head side is formed to be opened to a surface abutting against to a chain case, and then, each oil pressure supply port of the hydraulic control valve is opened in an oil passage boss integrally formed with the chain case to thereby bring the oil passage boss of the chain case into abutment with the abutment surface of the cylinder head side, and thereby communicate each oil passage on the cylinder head side with each oil pressure supply port of the hydraulic control valve.
There is a conventional engine provided with variable valve timing mechanism in which a plurality of hydraulic control valves are mounted onto a chain case in such a way that their axis lines are oriented to be normal to and overlapped with the crankshaft lines.
Furthermore, in a conventional variable valve timing mechanism-equipped engine, there is a merit of integrally forming the oil passage to the chain case. However, the conventional variable valve timing mechanism includes a defect or drawback in which when, for instance, a plurality of hydraulic control valves are installed for supplying working oil to an intake side hydraulic actuator attached to an intake camshaft and an exhaust side hydraulic actuator attached to an exhaust camshaft, respectively, it is difficult to achieve the compatibility with the layouts of valve train accessory (auxiliary equipment) driving parts and lubrication-and-cooling passages.
For this reason, a conventional variable valve timing mechanism-equipped engine tends to involve complicated arrangements such as 3-dimensional machining working of oil passages, addition of seal plugs, and so on in order to avoid interference with objects, thereby causing an increase in machining and management cost and a quality loss in the manufacturing process.
One conventional structure of a variable valve timing mechanism of an engine is disclosed, for example, in Patent Document 1 (Japanese Patent Laid-Open Publication No. 2008-215323). In the structure of this Patent Document 1, since the oil passages of the chain case side are configured to be in direct communication with the oil passages of the cylinder head side, it is difficult to freely curve or bend the oil passage between the chain case and the cylinder head during a machining process, which therefore limits in the forming of oil passages.
Furthermore, in the structure of the Patent Document 1, since hydraulic control valves are disposed only in a limited range or space of oil passages between the chain case and the cylinder head, the hydraulic control valves cannot be freely disposed to the chain case, which provides limitation in the attaching position of the hydraulic control valves. This degrades the mountability of the hydraulic control valve to the chain case.